Metronidazole Warfarin Interaction — ORE Part 1 MCQ

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HardPharmacology & TherapeuticsMetronidazole Warfarin InteractionORE Part 1

A patient taking warfarin presents with a spreading dental abscess requiring extraction and adjunctive antibiotic treatment. Their INR is 2.8, measured today, and they report an immediate hypersensitivity reaction to penicillin. Which antibiotic requires particular caution because it can potentiate the anticoagulant effect of warfarin?

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Correct answer: E — Metronidazole

E. Metronidazole is correct. Its UK SmPC specifically warns that it may potentiate warfarin-type oral anticoagulants; a warfarin dose reduction may be needed and prothrombin time/INR monitoring is advised. SDCEP similarly advises that, if an antibiotic is prescribed to a patient taking a vitamin K antagonist, INR may increase and the anticoagulation service should be informed. An INR of 2.8 permits extraction without stopping warfarin because it is below 4. Cefalexin, azithromycin and doxycycline may each require ordinary prescribing judgement, but none is the clearly specified warfarin-potentiating antibiotic tested here. Phenoxymethylpenicillin is unsuitable because of the stated immediate penicillin allergy, but that is a separate issue from the interaction being asked about.

Reference: Electronic Medicines Compendium. Metronidazole 200 mg Tablets – Summary of Product Characteristics, section 4.5, 2024. https://www.medicines.org.uk/emc/product/12348/smpc